Coding and interference suppression for CDMA systems on fading channels.

摘要

The performance of a turbo-coded direct-sequence code-division multiple-access (DS-CDMA) system with an adaptive, minimum mean-square-error (MMSE) receiver for interference suppression is analyzed on a Rayleigh fading channel. In order to accurately estimate the performance of the turbo coding, two improvements are proposed for the conventional union bound: the information of the minimum distance of a particular turbo interleaver is used to modify the average weight spectra, and the tangential bound is extended to the Rayleigh fading channel. Theoretical results are derived based on the optimum tap weights of the MMSE receiver and maximum-likelihood decoding. Both the analysis and simulation results show that in the majority of the scenarios that we are concerned with, the MMSE receiver with a rate-1/2 turbo code will outperform a rate-1/4 turbo code. They also show that, for a bit error rate lower than about 10−3, the capacity of the system is increased by using turbo codes over convolutional codes, even with small block sizes.; Motivated by these results, the performance of turbo-coded asynchronous DS-CDMA using long and short spreading sequences is also compared. The analysis shows that for a matched-filter (MF) receiver on either the additive white Gaussian noise channel or the flat Rayleigh fading channel, there is a degradation in the average performance of the turbo-coded short-sequence systems compared to the long-sequence systems due to the fact that the cross-correlations of the short-sequence systems are not time-varying. However, the MMSE receiver can be used in short-sequence systems to suppress the interference. Such a receiver in the turbo-coded system is shown to outperform the long-sequence system with the MF receiver, especially when there is a near-far problem. Similar results are obtained by computer simulations for the turbo-coded CDMA systems on a frequency-selective Rayleigh fading channel.; In the last part of this dissertation, the intersymbol interference (ISI) caused by the multipath fading channel is considered in a DS-CDMA system when the processing gain of the CDMA system is small. A new receiver based on maximum-likelihood sequence estimation (MLSE) is proposed, which performs the functions of despreading and equalization simultaneously. Accurate analytical bounds are derived for the bit-error probability when random spreading sequences are used. The numerical results show that the MLSE receiver can recover almost all the loss caused by the ISI effect.